The present invention relates to battery-powered systems or subsystems in which multiple integrated circuits must sustain data communications.
The power efficiency requirements of battery-powered modules can be extremely stringent. For best reliability, neither battery recharging nor user detection of imminent failure should be relied on. Therefore, to reliably meet the design battery lifetime of a module, extreme care must be taken to identify and control the worst-case battery-drain scenarios.
Battery lifetime management can be particularly difficult where several complex integrated circuits are included in a battery-powered module. Where significant control interactions, or even data exchange, may occur, the demands of such transactions must also be allowed for.
The present invention provides a battery-powered module architecture which is particularly advantageous where a serial data interface is used to link two (or more) integrated circuits within the module. Among the teachings of the present invention is a battery-powered system, where a first integrated circuit provides a secondary power supply to a second integrated circuit, and also provides data signals in a serial protocol. The first integrated circuit steps down the secondary power supply when the reset-bar signal is being driven high. The second chip goes active whenever its reset-bar input exceeds its battery-voltage input.
Preferably a scaled supply voltage output is provided by a data converter chip to downstream chips (including an access control chip, in the presently preferred embodiment). The access control chip (in the presently preferred embodiment) goes active whenever its RST* input exceeds its V.sub.BAT input. Since RST* can be driven only to the positive power supply level, the converter chip provides the access control with a reference voltage level which is lower than the power supply level. (However, to minimize power consumption, this reference voltage level is lowered only when necessary, i.e. when the RST* line is being driven high.) This reference voltage is connected to the V.sub.BAT input of the access control chip.
A further feature is that the switching transistors are arranged in a configuration such that two transistors would have to fail before the battery could receive uncontrolled charging current. This provides improved safety.